As this type of a hydraulic cylinder there is known, for example, such a hydraulic cylinder as shown in FIG. 4, in which a bottom 2 and a cap 3 are provided at both end portions of a cylinder 1, a piston 4 is inserted movably into the cylinder 1, a hollow piston rod 5 is also inserted movably into the cylinder 1 through the cap 3, the piston 4 defines a rod-side oil chamber 6 and a piston-side oil chamber 7 within the cylinder 1, the rod-side oil chamber 6 being in communication with a reservoir 9 in the piston rod 5 through an oil hole 8 formed in the lower portion of the piston rod 5, a pipe 10 extending into the reservoir 9 is mounted upright on the upper end portion of the piston 4, a check valve 12 for opening and closing the reservoir 9 and the piston-side oil chamber 7 is disposed in a through hole 11 formed in the piston 4, and the piston-side oil chamber 7 communicates selectively with a pump or a tank through an oil passage 13.
In the above hydraulic cylinder, a recess 14 is formed in the upper end of the piston 4, the lower end of the piston rod 5 is threadedly engaged with the inner periphery of the recess 14, and within the recess 14 the lower end of the pipe 10 is welded centrally to the upper end of the piston 4.
Although the above conventional hydraulic cylinder does not involve any special problem in point of function, there are the following inconveniences.
1 Firstly, since the pipe is welded within the recess groove, the working efficiency of welding is low and it is difficult to remove spatter and scale resulting from welding. Residual spatter and scale may give rise to a serious trouble.
2 Secondly, the oil present in the rod-side oil chamber 6 and that in the reservoir 9 flow out through the oil hole 8 which is throttled gradually with the cap 3 during extension of the piston rod, to exhibit a cushioning effect. However, since the base end of the piston rod 5 is fitted in the recess 14, the oil incapable of being used for the cushioning effect stays in a dead space A formed between the outer periphery of the lower end of the pipe 10 and the inner periphery of the lower end of the piston rod 5, with the result that oil comes to nothing. In the above conventional hydraulic cylinder, moreover, since the cushioning effect is not exhibited in the vicinity of the most compressed state, it is possible that there will occur a sudden shock at the time of compression and the resulting fall of goods.